With the new M3 being a naturally aspiring 4.0 liter putting out 420 HP, I don't see much potential to extract much more power out of it (BMW's engineers goal is to get above 100 HP per liter and they got 105, so I don't see much left). The other issue is with the high compression ratio required to get those #'s, you would have to give some serious $$$$ if you wanted to go forced induction.